A so called charging/discharging technique is generally becoming popular in which power is supplied from such a power supplying apparatus as a battery charger to a charging/discharging apparatus represented by a battery pack used in a video camera or the like to charge the same.
As a recent battery pack, a battery pack of such a type where a microcomputer is incorporated therein and the charging capacity of the battery pack is memorized or stored in the computer has been provided for a practical use. In the battery pack of this type, the computer causes a built-in RAM (Random Access Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), a ROM (Read Only Memory) or the like to store data or information on the charging capacity of a battery pack now on use. When this battery pack is set to a video camera or a battery charger, the microcomputer reads out the stored information on the charging capacity to transmit the information to the video camera or the battery charger. On the other hand, the operation time or the charging time of the battery pack is calculated on the basis of this information and displayed in the video camera or the battery charger.
Regarding the power charged in a battery pack, however, it has been known that, even though the battery pack which has just charged had a charging capacity from which an allowable operation time of 120 minutes is calculated, for example, as shown in FIG. 1, when the battery pack is left in a non-use state for a long time, the battery pack gradually self-discharges. For example, when the charged battery pack is put in a non-use state for six months, the charging capacity of the battery pack stored in a memory is 120 minutes but its actual charging capacity is reduced to about 10% of the charging capacity stored in the memory.
In general, the allowable operation time of a battery pack has a relationship with the charging voltage of the battery pack shown in FIG. 2. That is, in case of the charging capacity of a battery pack which can drive a video camera for 120 minutes at maximum, for example, indication of about 8.4V shows a full charged state (100% charged state), and a time display of 120 minutes is performed as the allowable operation time. Then, the charging voltage and the allowable operation time decreases as an operating time elapses. Here, a pre-end voltage Vpre (Voltage Pre-end) (for example, 6.7 V) in FIG. 2 is a voltage value indicating that only a short allowable operation time is left. That is, when the allowable operation time becomes zero, there may occurs a problem that a saving operation can not be performed safely during execution of various processing. Under such circumstances, the pre-end voltage Vpre is set for warning a user in a state the allowable operation time is still left before the charging voltage reaches an end voltage Ve (Voltage end) (for example, 6.5V: a voltage at which a video camera or the like can not be driven) in order to prompt the user to perform a saving operation safely.
However, when the battery pack fully charged is kept in a non-operation state for a long time, the relationship between the charging time and the allowable operation time varies due to self-discharging of the battery pack, for example, as shown in FIG. 3. That is, since the charging voltage is changed to a lowered state (the charging capacity is reduced) due to the self-discharging, which results in reduction in actual allowable operation time. Even in this state, however, since the allowable operation time at a full charged state has been stored in the RAM, there is a problem that, though an actual allowable operation time is 30 minutes or so, the allowable operation time indication displays about 80 minutes, which causes an error. Incidentally, the allowable charging capacity has been actually stored in the RAM, and the allowable operation time is computed in the battery charger or the like on the basis of the allowable charging capacity to be displayed.
Also, as a method for detecting a self-discharge of a battery pack in case that the battery pack is left for a long time without using the same, there is a method where the state of a battery pack while it is not operated is monitored by a microcomputer in the battery pack and a self-discharged amount corresponding to the monitored state is reduced from the original charged state so that an error in an allowable operation time display of the battery is corrected. At this time, a power source for driving the microcomputer in the battery pack uses the power of the battery pack.
However, in case that the self-discharged amount due to the long term non-use is detected by the microcomputer in the battery pack, there occurs a case that a power more than that discharged due to the self-discharging of the battery pack is consumed for driving the microcomputer, and therefore a problem occurs that a non-use time of the battery pack must be significantly reduced.
Furthermore, there is a problem that an over-discharged state occurs in that the voltage of the battery pack is discharged below a voltage level required for maintaining functions of the battery pack, which injures a safety or a reliability of the battery pack.
As set forth above, it is considered to be desirable that the microcomputer in the battery pack is controlled to a non-operation state (microcomputer sleep) in a state that the battery pack is not used (a state that charging and discharging are not conducted).
The present invention has been made in view of these circumstances, and an object thereof is to perform correction such that an allowable operation time of a battery pack can be displayed accurately.